Parallel plate microwave applicator

ABSTRACT

Microwave dielectric heating is finding greater acceptance in industrial processing. In particular, the parallel plate waveguide applicator is useful in the heating of thin films or webs. However, for materials which have a very small dielectric loss factor and thickness product, the parallel plate applicator does not provide sufficiently efficient coupling of microwave energy to the web. The present invention provides an applicator of improved efficiency and allows flexibility of choice of coupling along the length of the applicator. This is accomplished by inserting inductive elements of predetermined dimensions such as metallic rods or plates, at predetermined positions within the waveguide, providing for multiple electric reflections of the excitation field between the elements. Also by varying the susceptance of these elements, which includes varying the size and/or locations and the spacing of the elements or by inserting dielectric material into the guide, the coupling of microwave energy to the web may be varied and thus controlled over the length of the applicator.

United States Patent r191 VanKoughnett 1 Nov. 26, 1974 1 1 PARALLEL PLATE MICROWAVE APPLICATOR Allan L. VanKoughnett, Ottawa, Ontario, Canada [75] Inventor:

[73] Assignee: Canadian Patents and Development Limited, Ottawa, Ontario, Canada [22] Filed: Dec. 10, 1973 [21] Appl. N0.: 423,551

[52] US. Cl 219/1055 A, 219/1055 E [51'] Int. Cl. H05b 9/06 [58] Field 01' Search 219/1055, 10.61

11/1969 Schroeder 219/1055 ll/l973 Gerling 219/1055 Primary Examiner-J. V. Truhe Attorney, Agent,.0r Firm-Edward Rymek 57 ABSTRACT Microwave dielectric heating is finding greater acceptance in industrial processing. In particular, the parallel plate waveguide applicator is useful in the heating of thin films or webs. However, for materials which have a very small dielectric loss factor and thickness product, the parallel plate applicator does not provide sufficiently efficient coupling of microwave energy to the web. The present invention provides an applicator of improved efficiency and allows flexibility of choice of coupling along the length of the applicator. This is accomplished by inserting inductive elements of predetermined dimensions such as metallic rods or plates, at predetermined positions within the waveguide, providing for multiple electric reflections of the excitation field between the elements. Also by varying the susceptance of these elements, which includes varying the size and/or locations and the spacing of the elements or by inserting dielectric material into the guide, the coupling of microwave energy to the web may be varied and thus controlled over the length of the applicator.

12 Claims, 8 Drawing Figures PATENIEL HCIV 2 6 I974 SHEET 3 BF 5 7 PARALLEL PLATE MICROWAVE APPLICATOR This invention relates to microwave applicators for subjecting workpieces to dielectric heating and in particular parallel plate waveguide applicators which provide improved coupling of the electric field and the workpiece, which is in the form of a thin sheet material or a web, as it moves through the applicator.

Microwave heating systems are finding increasing acceptance as efficient, economical means of industrial processing. One of the most important classes of microwave systems concerns heating of thin sheet materials or webs. Most existing and potential applications in this class involve drying the web or drying thin coatings applied to the web. A typical application of this nature is drying a thin coating of glue applied to a paper web in the manufacture of remoistenable glued paper tape.

A microwave heating chamber or applicator for thin webs should make efficient use of the available microwave power and should distribute the microwave energy to uniformly heat the web.

One such system is described in US. Pat. No. 3,457,385 issued July 22, 1969 to W. A. Cumming, assignor to Canadian Patents and Development Limited. In this system, microwave energy in the form of a single travelling wave is propagated along an electrically continuous waveguide, the microwave energy being of an operating mode, usually the TE mode, that yields a concentration of lines of electric force in a certain area of the cross-section of the waveguide. The thin film work piece is fed through the longitudinal axis at the centre plane of the waveguide where the concentration of the electric field is greatest.

The above parallel plate waveguide applicator is excited by a uniform electric field which is derived from an appropriate feed device. The applicator may be employed in resonant or non-resonant systems. In the resonant configuration, energy passes from a microwave source, through a tuner and feed assembly to the applicator and propagates along the workpiece. Any energy not absorbed on this first pass along the workpiece is reflected by a slotted shorting plate which forms the workpiece exit port and propagates back along the workpiece toward the tuner. The portion of this energy not absorbed by the workpiece reaches the tuner. When the tuner is properly adjusted this energy returned from the applicator is reflected at the tuner such that all of the power generated by the source is forced to be dissipated in the heating chamber. Standing waves thus exist along the direction of workpiece transport but field uniformity across the workpiece is maintained so long as only the single uniform mode is excited in the applicator by the feed assembly.

The parallel plate applicator can also be employed in a non-resonant configuration. If the slotted shorting plate on the end of the applicator is replaced by a nonreflecting water load and the tuner removed, a simple non-resonant system results. Alternately, the slotted shorting plate can be retained and the tunerbe replaced by a circulator to achieve a non-resonant system of higher efficiency. The choice of system for a particular application depends upon the properties of the web. In general, low loss webs dictate a resonant system in order to achieve a respectable efficiency of conversion of microwave energy to heat in the web. The parallel plate waveguide applicator operating in a resonant mode has proven advantageous when the web is relatively thin and not particularly lossy and when good uniformity of heating and efficiency are required. However it becomes impracticalwith webs for which the product of web dielectric loss factor and web thickness is very small. When non-resonant applicators are employed with such webs, the efficiency of conversion of microwave energy to heat in the web becomes quite low. With resonant applicators, respectable efficiencies maybe realized but other problems with operation of a very high Q resonator are likely. In the case of a parallel plate chamber heating a very low loss web. tuner adjustment becomes critical and the possibility ol'tuning in a higher order mode with a non-uniform heating pattern is increased. To avoid the need of frequent operator attention the tuning mechanism can be automated but in the interest of avoiding stringent tuner requirements, insuring uniform heating, and reducing radiation loss, operation with a lower system Q and hence stronger coupling to the web is desirable.

It is therefore an object of this invention to provide an applicator which will exhibit improved coupling between the electric field and a workpiece having a small dielectric loss factor.

It is a further object of this invention to provide an applicator which will uniformly heat workpieces in the form of thin sheets or continuous thin films or webs.

It is another object of this invention to provide an applicator in which the amount of coupling is adjusted over the length of the applicator.

It is a further object of this invention to provide an applicator which includes support for the workpiece as it moves therethrough.

These and other objects are generally achieved in a microwave applicator which includes a rectangular waveguide having a cross-sectional width A and height B. The waveguide may be straight or curved along its length. The dimension A is greater than B and is determined by the width of the workpieces such as the sheets, webs or films which are fed through the waveguide along its length. The applicator is excited by a microwave source through a feed section principally in the TE, mode which provides a uniform electric field intensity across the width of the waveguide. In order to increase coupling between the electric field and the workpiece to the heated, multiple electrical reflections ar created between elements mounted within the waveguide. This increases the field intensity between the elements in certain regions along the height of the guide. In the preferred embodiments, these elements are inductive and may be either metal plates or rods extending across the width A. The elements are spaced along the length of the waveguide, but symmetrically located about a plane through the center of the width A.

In one preferred embodiment, rods are mounted at some predetermined location above or below the centerline of the height B. However, if maximum coupling between the microwave energy and the web is desired, the rods are mounted near the centerline of height B such that the workpiece may move along the centerline. In addition, the field intensity may be adjusted over the length of the waveguide by varying the spacing between rods which have different diameters or by inserting a dielectric material such as Teflon having varying thicknesses within the waveguide.

In addition, the rods may be mounted using choke joints and bearings allowing the rods to rotate freely or to be driven by an exterior drive.

Finally, it may be preferred to place a coating or sleeve of dielectric material such as Teflon over the rods which will protect the workpiece from being scratched or marred and which allows greater freedom in locating the rod within the waveguide.

In the drawings:

FIG. 1 is a schematic view of a basic prior art system for dielectric heating.

FIG. 2 is a side view of one embodiment of the invention including inductive rods in the applicator.

FIG. 3 is a cross-sectional view of the applicator taken along lines a-a in FIG. 2.

FIG. 4 is a view of an embodiment of the invention including dielectric material in the applicator.

FIG. 5 is a side view of an embodiment of the invention including unevenly spaced inductive rods.

FIG. 6 is a side view of a portion of the applicator in which the inductive rods are free to roll.

FIG. 7 is a cross-section view of the applicator taken along lines bb in FIG. 6.

FIG. 8 is a view of an embodiment of the invention having a curved applicator.

FIG. 1 illustrates a basic prior art system developed for subjecting workpieces, such as thin films or webs, to dielectric heating. It includes a parallel plate waveguide applicator 1 wherein the narrow dimension of the waveguide is expanded to accomodate the width of the web. The expanded waveguide is excited in principally the TE mode with a constant electric field across the width of the waveguide. A microwave source 2 provides power to feed section 4 through a connecting waveguide 3. Several feed sections have been developed which will excite a uniform electric field across the width of the applicator I. This field is represented by arrows 5 which show a uniform field across the width of the waveguide l, but one which decreases sinusoidally in strength toward the top and bottom of the waveguide. Hence, the field is of greatest strength in the plane through the center of the height of the waveguide.

The web or film 6 is therefore moved through the applicator in this central plane to achieve maximum coupling with the field resulting in dielectric heating in the workpiece.

However, it has been determined that if the uniformity of the applicator along its length is disturbed appropriately, improved efficiencies would result. FIGS. 2 and 3 illustrate an applicator l in which an electric field is excited through a feed section 4 from a microwave source (not shown). The applicator includes metal rods 7 of equal diameter, which are equally spaced throughout the length of the applicator and from the face 8 of the chamber. These rods may be mounted by conventional mounting devices. The rods 7 may be replaced by other inductive elements, such as metal plates, which will provide the same result.

Rod 7 is located so as to provide matching between the open section of the applicator and the section including the rods. As seen in FIG. 3, the rods 7 or other inductive elements are symmetrically located about the plane A through the center of dimension A and are of length A. In FIG. 3, the rod is located slightly below B,,, a plane through the center of dimension B such that the workpiece 6 may be supported along B for maximum coupling.

If however, less coupling is required, rods 7 may be mounted in a plane located at some predetermined distance above or below plane 8,, such that the workpiece will not move along plane B,,. When non-uniform coupling is desired along the length of the applicator, the rods may be mounted in a plane which is at a slight angle to plane B,,. This provides for increased coupling as the workpiece moves from one end of the applicator to the other.

For workpieces which require a compressional force while they are being heat treated, such as for drying glue which bonds two webs or films, the equally spaced rods may be alternately located on one side and the other of the plane 8,, with the workpiece looped in tension over the rods, compressing the webs or films together.

The operation of the applicator with rods included may be analysed by representing the rods by a shunt inductance in an equivalent circuit of the waveguide. A section of the waveguide between two identical rods spacd a distance L apart is taken and it is assumed that a signal of unit amplitude is incident on the rods from one side with the output terminated in a matched load.

The waveguide section between the rods will contain waves propagating in both directions. If the rods spacing L and equivalent rod susceptance B are chosen such that the structure is matched at the input port, the wave amplitudes a and b are given by for an incident wave of unit amplitude. The waves a and b interfere constructively and destructively to produce a standing wave pattern between the rods with maxima and minima of [1 3 /41" 3/2 and [l 8 /41 8/2. The average value of the square of the electric field over one period of the standing wave is whereas the electric field has unit amplitude outside the length of waveguide between the rods. When a thin sheet of low loss material is inserted in the waveguide parallel to the electric field, the energy dissipated in the sheet which is proportional to the average value of the square of the electric field, has been increased through introduction of the reactive elements. Consequently, increased coupling to a sheet material can be achieved if it is situated between the reactive elements as compared with the case in which no reactive loading is employed. Furthermore, the degree of coupling can be controlled through the value of the shunt susceptance B.

The electric field vanishes on the surface of each rod but regains the sinusoidal distribution between the top and bottom faces of the applicator between the rods. Also the electric field remains uniform across the width A of the chamber.

The coupling of the field to the web can be controlled in various ways other than described with regard to FIGS. 2 and 3.

FIG. 4 illustrates an applicator 1 as described in FIG. 2 having equal diameter rods 7. The applicator 1 is loaded with a dielectric material 9, such as Teflon. The material which may be mounted on either or both of the top and bottom surfaces, has a uniform thickness across the width A of the waveguide and non-uniform thickness along the length. The loading of the applica tor in this manner varies the effective electrical separation of the rods in a non-uniform manner. This is desirable, for example, when drying a very wet workpiece such as a web to near bone dry. Strong coupling to the web is not required as the wet web enters the applicator, but it useful to increase the coupling near the dry end to improve the efficiency.

Non-uniform reactive loading can also be realized by successively increasing the diameter of the rods as well as the spacing of the rods along the length of the applicator. This is illustrated in FIG. 5 wherein the rods 7a, 7b, 7c, 7d, 7e and 7f as well as the spacings between these rods increase in size from left to right to provide greater coupling on the right end.

A further embodiment of the invention which can be applied to the configurations previously described is illustrated in FIGS. 6 and 7. The rod 7 is extended through openings 10 in each side of the applicator, and is mounted on bearings 11 which are fitted in choke joints 12. The choke joints may be constructed in any conventional manner to prevent leakage along rod 7 to the exterior of the applicator l. The rods 7 may therefore rotate freely as the workpiece, supported by the rods, is moved through the applicator. In addition, rods 7 may have extensions 13 by which all of the rods may be rotated in synchronism with the movement of the workpiece to prevent damage by slippage between the rod and workpiece. Alternately the rods themselves may move a workpiece through the applicator, such as a thin rigid material which is too short to be pushed or pulled through from either end.

A coating or sleeve 14 may also be placed on the section of the rod which is within the applicator. This sleeve, made from a dielectric material such as Teflon, polyethelyne or polystyrene, allows more freedom for the effective placement of the rods, and prevents the workpiece from being damaged by the metal rods. Though described with respect to the rotatable rods, the dielectric may also be placed on the fixed rods.

Another modification to the applicator which may also be applied to the applicators described above is illustrated in FIG. 8. The applicator 1 includes a waveguide which is curved along its length. The spaced rods 7 which may be fixed or rotatable thus support a tensioned workpiece 6 and prevent it from curling across 'its width. This applicator is particularly useful for drying materials in which the base and coating have different coefficients of thermal expansion.

I claim:

1. A microwave applicator for subjecting a workpiece to dielectric heating as it moves therethrough,

6 comprising:

a. a rectangular waveguide of cross-sectional dimensions A and B where A B; b. means for exciting said waveguide principally in the TE mode to provide a substantially uniform electric field intensity across the dimension A; and

c. at least first and second means mounted within the length of said waveguide and suitably spaced to provide an electric field of increased intensity through multiple reflections between said mounted means.

2. An applicator as claimed in claim I in which said mounted means comprise inductive elements.

3. An applicator as claimed in claim 2 in which said inductive elements include elongated inductive rods; means adapted to mount said rods symmetrically about a plane through the center of the dimension A; and said rods extending across the dimension A.

4. An applicator as claimed in claim 3 in which said rods further include a low loss dielectric coating.

5. An applicator as claimed in claim 4 in which said dielectric coating is teflon.

6. An applicator as claimed in claim 3 in which said rods are of equal diameter D; and said rods are equally spaced in a plane parallel to and a distance of approximately one-half D from a plane through the center of dimension B.

7. An applicator as claimed in claim 6 which further includes a dielectric material mounted on at least one of the waveguide interior surfaces of dimension A;

said dielectric material being uniform across dimension A and successively thinner along the waveguide length to successively vary the effective electrical separation of said rods.

8. An applicator as claimed in claim 3 in which said rods are of equal diameter D; and said rods are uniformly spaced along the length of said waveguide, alternately to one side and the other side of a plane through the center of dimension B.

9. An applicator as claimed in claim 3 in which successive spacings between the rods increases along the length of the waveguide.

10. An applicator as claimed in claim 3 in which the diameter of successive rods decreases along the length of the waveguide.

11. An applicator as claimed in claim 3 in which the rods are rigidly fixed to the waveguide walls of dimension B.

12. An applicator as claimed in claim 1 in which said 

1. A microwave applicator for subjecting a workpiece to dielectric heating as it moves therethrough, comprising: a. a rectangular waveguide of cross-sectional dimensions A and B where A>B; b. means for exciting said waveguide principally in the TE01 mode to provide a substantially uniform electric field intensity across the dimension A; and c. at least first and second means mounted within the length of said waveguide and suitably spaced to provide an electric field of increased intensity through multiple reflections between said mounted means.
 2. An applicator as claimed in claim 1 in which said mounted means comprise inductive elements.
 3. An applicator as claimed in claim 2 in which said inductive elements include elongated inductive rods; means adapted to mount said rods symmetrically about a plane through the center of the dimension A; and said rods extending across the dimension A.
 4. An applicator as claimed in claim 3 in which said rods further include a low loss dielectric coating.
 5. An applicator as claimed in claim 4 in which said dielectric coating is teflon.
 6. An applicator as claimed in claim 3 in which said rods are of equal diameter D; and said rods are equally spaced in a plane parallel to and a distance of approximately one-half D from a plane through the center of dimension B.
 7. An applicator as claimed in claim 6 which further includes a dielectric material mounted on at least one of the waveguide interior surfaces of dimension A; said dielectric material being uniform across dimension A and successively thinner along the waveguide length to successively vary the effective electrical separation of said rods.
 8. An applicator as claimed in claim 3 in which said rods are of equal diameter D; and said rods are uniformly spaced along the length of said waveguide, alternaTely to one side and the other side of a plane through the center of dimension B.
 9. An applicator as claimed in claim 3 in which successive spacings between the rods increases along the length of the waveguide.
 10. An applicator as claimed in claim 3 in which the diameter of successive rods decreases along the length of the waveguide.
 11. An applicator as claimed in claim 3 in which the rods are rigidly fixed to the waveguide walls of dimension B.
 12. An applicator as claimed in claim 1 in which said waveguide is curved along its length. 